European Patent Application EP 2 018 032 & International Patent Application WO 2009/014438 (TNO/KPN) disclose a method of identifying proximate mobile devices. This known method comprises the steps of providing a sensory identifier capable of being detected by the human senses, at least two mobile devices detecting the sensory identifier, one of the mobile devices sending a message to another mobile device, and the other mobile device sending an acknowledgement so as to establish mutual identification. The sensory identifier may be a visual identifier, such as a bar code, or an audio identifier, such as a sound signal. The method of WO 2009/014438 may involve a server for carrying out (part of) the identification process, and/or for carrying out the optional step of verifying a sensory identifier so as to reduce the probability of erroneous identifications.
Erroneous identifications may occur, for example, if several devices detect similar sensory identifiers. A certain sound signal, such as the sound produced by tapping two (mobile) devices together, may be detected by devices that are not involved in the tapping. Erroneous identifications may also occur when two or more similar sensory identifiers are produced almost simultaneously in the proximity of the devices, for instance by tapping together several pairs of (mobile) devices in the same room at approximately the same instant.